Solenoid operated fuel injectors are used to inject fuel into the cylinder of internal combustion engines. A solenoid actuator of the solenoid operated fuel injector is energized to move a control valve element in a first direction to initiate an injection event and the actuator is de-energized to allow the control valve element to move in an opposite direction to end the injection event. In order to improve fuel economy and reduce emissions, fuel injection systems must be capable of achieving high injection pressures, controlling injection rates, and providing fast responses while maintaining accurate and reliable control of fuel metering and injection timing functions.
The ability of a fuel injector to respond to an input signal command to open significantly effects the ability of the fuel injector to deliver a precise injection of fuel to the combustion chamber. Parameters that define the fuel injector's magnetic circuit (e.g., the stator, the armature, and the working gap between the stator and armature) are particularly important since it is the magnetic circuit that conducts the magnetic flux that exerts the magnetic force which acts on the armature. The rate at which the magnetic flux builds determines the rate at which force acting on the armature builds. The faster the force builds, the faster the fuel injector responds. Additionally, minimizing the size of the solenoid actuator of the fuel injector is desirable, especially where the valve is mounted inside a fuel injector body.
Eddy currents play a significant role in the magnetic circuit and reducing eddy currents aid in faster response time of the fuel injector. For example, many stator cores are formed of a laminate stack assembly which permits faster magnetization and demagnetization of the solenoid by breaking up eddy current paths thereby reducing eddy currents.
Efforts have been made to minimize the size of solenoid actuators while providing the response time required in high speed, high pressure applications. For instance, the attractive force of the stator assembly of a solenoid actuator assembly can be increased by increasing the surface area of the stator pole end faces. The end face may be increased by sizing and shaping the stator assembly to occupy a maximum amount of the space in a surrounding housing. Nevertheless, the relatively small gap between the inner diameter of the housing and the outer diameter of the stator causes flux leakage into the surrounding housing. Generally, sizing and shaping the stator assembly to occupy a maximum amount of space in a surrounding housing requires designing the inner diameter of the housing and the outer diameter of the stator to very close tolerances.
Various solenoid assembly designs that increase attractive forces, reduce eddy currents and reduce flux leakage have been developed. One such example is described in U.S. Pat. No. 6,155,503 (the '503 patent) issued to Benson et al. on Dec. 5, 2000. The '503 patent includes a solenoid stator assembly positioned in an actuator housing and a flux dissipation reducing feature to minimize flux leakage into the housing and thus maximize the attractive force, which in turn improves valve response time. The flux dissipation reducing feature disclosed in the '503 patent includes a slot formed in the housing adjacent each outer face of the solenoid stator pole pieces. The slots permit the cross sectional area of the pole pieces to be maximized thereby increasing the available attractive force. In addition, the slots increase the resistivity of the magnetic circuit and reduce eddy currents.
The apparatus of the '503 patent may not adequately reduce the gap between the stator and the surrounding housing. Furthermore, the design of the '503 patent may require tight tolerances for a close fit of the stator within the housing, which may make manufacturing the design expensive. In addition, the design disclosed in the '503 patent only applies to E-type laminate stack assemblies, and other stator designs would not benefit. In particular, it may not be practical to incorporate the slots from the E-type laminate stack in other stator designs and thereby reduce eddy currents. Thus, the system described in the '503 patent may be ineffective in situations where a non E-type laminate stack stator is required, in situations where the gap between the stator and the surrounding housing must be further reduced, and in situations where eddy currents must be reduced.